The present invention relates to apparatus for the coding and decoding of electrical signals within a pre-established frequency band, for the protection of communication secrecy, which may be connected between means for the generation and reception of the electrical signals and means for the exchange of the coded signals with at least one other coding and decoding apparatus.
Communication systems are known which code a transmitted signal and decode a received signal, generally a signal within the voice band, which provide for the protection of communication secrecy by acting either in the frequency domain by inversions and translations of the transmitted signal or in the time domain by sampling and temporarily storing in memory subsequent fragments of the transmitted signal and sending later these fragments on a communication line according to a pseudo-random order. More particularly, in this latter type of apparatus, the phonic signal, after sampling, is generally converted from analog-to-digital form and sent to different logic memory registers. From these registers the signal, after extraction and reconversion from digital to analog form, is sent on the communication line through a control proceeding from a code generator which selects the registers according to a variable sequence having a repetition period very high with respect to the short lapses of time encountered in a radio of telephonic communication, so that the selection is substantially random.
At the present time, apparatus of this type, while appreciably improving the secrecy of communications present some drawbacks which limit performance. As examples, first, the code generator has no control over the register selection sequence, and hence the same register may be selected many times in succession, the fragments of the phonic signal contained in the register being consequently transmitted in their natural order, and the signal transmission may thus be "clear" for short periods. Second, to allow a correct decoding of the transmitted signal, the code generator of the receiving apparatus must necessarily be synchronized with the code generator of the transmitting apparatus with which it is communicating. This synchronizing operation is repeated, in known apparatus, each time it is desired to reverse the communication sense between the users; therefore, in a transmission system with more than two units in different places connected at the same time, the non-reception of a synchronizing signal will cause the disabling for reception of one or more receiving units. Third, the changing from the transmission of a non-coded ("clear") signal, to the transmission of a coded signal requires manual operations for the adaptation of the units by all users, so that use of the units is not simple. Fourth, known units essentially operate in simplex, i.e., with a single active transmission and reception channel, even if the communication line is of the duplex type and would allow the simultaneous communication in the two directions of the line. Hence, it would be particularly advantageous to utilize such a line in a complete way, to permit duplex communication similar to that between two conventional telephones. Finally, such known apparatus is relatively costly, due to the number and the cost of individual electronic components used, and hence the widespread use of such systems is limited.
Accordingly, an object of the present invention is to provide coding and decoding apparatus, operating in the time domain, for the protection of communication secrecy, which obviates the above drawbacks and permits further frequency coding of a phonic signal, which is easily and quickly installed and simple to use, particularly in a telephone line or radio relay link, and which may be provided as a portable unit which results in cost reduction compared with units presently available, while still maintaining high security and good communication quality.
According to the present invention, there is provided apparatus for the coding and decoding of electrical signals within a pre-established frequency band for the protection of communication secrecy, which may be connected between means for the generation and reception of the electrical signals and means for the exchange of the coded signals with at least one other coding and decoding apparatus. Such apparatus comprises a transmission and coding circuit and a reception and decoding circuit for the signals, with an operation complementarily enabled by commutation means. The circuits include first means, preferably having a plurality of memory elements, for temporarily storing consecutively received portions of the electrical signals, and second means for enabling operation of the first means to consecutively store in memory the signal portions and to send them, according to a different order, to the exchange means or to the means for the reception of the signals. Third means may be included for storing the electrical signals portions within the first means according to a fixed sampling frequency or to two or more variable frequencies in a pre-established order.
Presently preferred embodiments of the invention will now be described, only as an example, and with reference to the accompanying drawings.